There is known a photodetector in which a plurality of avalanche photodiodes (APDs) are arranged as photodetecting elements in a single pixel. As a representative photodetector, a silicon photomultiplier (SiPM) using silicon diodes as APDs is known.
For the silicon diodes, generally, p-n diodes are used. In the SiPM, a plurality of units, each including an APD and a quenching resistor, are connected in parallel in a single pixel. When a reverse-bias voltage is applied to an APD, a depletion layer is formed in the APD. Within the depletion layer, an electric field is formed by ionized donors and acceptors. When electron-hole pairs are formed within the depletion layer by the electric field, the electrons and holes move in opposite directions. When the electric field is strong, the moving speeds of the electrons and holes increase.
When the electrons and holes move within the depletion layer, they repeat collisions with atoms constituting the semiconductor. When the speeds of the electrons and holes exceed values determined for the electrons and holes, respectively, other electron-hole pairs are newly formed upon the collision with atoms constituting the semiconductor.
The new electron-hole pairs are also accelerated by the electric field and collide with other semiconductor constituent atoms, further forming other electron-hole pairs. By a chain reaction where such a phenomenon is repeated, a large avalanche breakdown current flows, triggered by the formation of one electron-hole pair. The function of the quenching resistor is to stop the avalanche breakdown current after a certain current flows. Specifically, by the avalanche breakdown current flowing through the quenching resistor, a voltage drop occurs at both ends of the quenching resistor. As a result, the voltage applied to the pn junction of the APD decreases and then the strength of the electric field within the depletion layer decreases, stopping the avalanche breakdown. The gain of the APD at this time is as very high as 105 to 106, and thus, even weak light of a single photon can be measured.
In addition, there is disclosed a device in which a plurality of combinations, each having a plurality of APDs and a scintillator that converts X-rays into scintillation light, are arranged. By thus combining APDs and a scintillator, an image having a spatial resolution according to the size of the scintillator can be obtained using photo-counting technique. For example, there is also known a technique for obtaining a CT (Computed Tomography) image by detecting X-rays.
As such, the SiPM can obtain high sensitivity by using the avalanche breakdown. However, the sensitivity of the SiPM greatly changes due to minute fluctuations in the voltage that is applied during a drive period in which photocurrents are obtained. Further, the sensitivity of the SiPM is sensitive to changes in temperature. In addition, when a voltage is applied to the photodetecting elements, Joule heat is generated. In view of this, JP-A 2013-16638 (KOKAI) discloses a technique for adjusting the voltage value of a voltage applied during the drive period, according to the temperature of an SiPM. In addition, JP-A 2011-258645 (KOKAI) discloses a technique for adjusting a voltage value applied during the standby period in order to suppress the influence of a dark current measured during the drive period.
Here, the present inventors have found that, when a drive voltage that is applied during a drive period in which photocurrents are obtained is continuously applied to a photodetecting element, the avalanche breakdown voltage of the photodetecting element changes. If the avalanche breakdown voltage is changed, then an appropriate drive voltage value at which the maximum sensitivity can be obtained in the photodetecting element changes. Namely, the conventional art has a problem that by continuously applying, as a drive voltage, a reverse-bias voltage higher than the avalanche breakdown voltage of the photodetecting element, the sensitivity of the photodetecting element decreases.